I do not think that it would be a problem, market cappitalization doesn´t mean that such amount of money is put into the Ethereum. Market cappitalization is calculated on the price and number of tokens. And if you think about it, Tether wouldn´t fight with Ethereum because Tether can´t work without the Ethereum network. Of course that they can move into other chain, but Ethereum has the most active users and it is user friendly.
Experts explain the further growth of the market share of Tether by its wide acceptance in the cryptocurrency industry and its connection with the dollar — the world's reserve currency. Tether might take the second place after BTC, but I do not think that it will affect the significance of ETH. Actually, almost nothing will change. USTD has simply become the simplest and very convenient tool for accessing Bitcoin.